A requirement for increased capacity in the U.S. cellular radio system has resulted in adoption of digital technology. The digital system employs Time Division. Multiple Access (TDMA) as a channel access method in conjunction with a digital modulation scheme. A proposed IS-54 standard for digital cellular radio specifies a particular frame and slot structure. Under this standard, three to six users share a common 30 KHz radio frequency (RF) channel. Each user transmits data in an assigned time slot which is part of a larger frame. The gross bit rate of data transmitted over the channel is 48.6 Kbits/sec. The transmitted digital data is first mapped onto Pi/4-shifted-Differentially Encoded Quadrature Phase Shift Keying (DQPSK) symbols and then pulse shaped using a square root raised cosine filter. The pulse shaped signal is subsequently modulated onto an RF carrier.
Data transmission in this digital cellular system is adversely affected by multipath propagation which causes delay spread, and consequent Inter-Symbol Interference (ISI), where a symbol is comprised of a pair of binary bits. Delay spread is expressed in terms of a quantity called delay interval which is measured as the time interval between the first ray and last significant ray arriving at the receiver. Delay spreads exceeding one third of the symbol duration cause a significant increase in Bit Error Rate (BER), necessitating use of an equalizer in the receiver. Typical delay spreads encountered in urban and rural areas in the U.S. are less than 40 microseconds, implying a need for equalization of one symbol of interference (40 microseconds)for a data rate of 48.6 Kbits/sec. Mobile receivers also experience rapid channel variations and Doppler induced frequency shifts that are proportional to vehicle speed.
The channel impairments described above require that nonlinear adaptive equalizers be incorporated in cellular radios. Two suitable equalizers are the Decision Feedback Equalizer (DFE) and an equalizer based on Maximum Likelihood Sequence Estimation (MLSE). The MLSE method employs the well known Viterbi algorithm and is referred to in the art as a Viterbi Equalizer or an MLSE-VA equalizer.
Both the MLSE and DFE techniques have been researched in some detail for use in the European CEPT/GSM cellular radio system. Results of this research are reported by, for example, R. D'Avella et al., "An Adaptive MLSE Receiver for TDMA Digital Mobile Radio", IEEE Journal on Selected Areas in Communications, Vol. 7, No. 1, pp. 122-129, January 1989, G. D'Aria et al., "Design and Performance of Synchronization Techniques and Viterbi Adaptive Equalizers for Narrowband TDMA Mobile Radio", proceedings of 3rd Nordic Seminar on Digital Land Mobile Radio Comm., Copenhagen, Denmark, September 13-15, and A. Baier et al., "Bit Synchronization and Timing Sensitivity in Adaptive Viterbi Equalizers for Narrowband TDMA Digital Mobile Radio Systems", proceedings of IEEE Vehicular Technology Conference, Philadelphia, pp. 372-382, 1988. The following two references also relate to DFE for the CEPT/GSM system: G. D'Aria et al., "Adaptive Baseband Equalizers for Narrowband TDMA/FDMA Mobile Radio", CSELT Technical Report, Vol. 16, No. 1, pp. 19-27, February 1988; and G. D'Aria et al., "Results on Fast-Kalman Estimation to Adaptive Equalizers for Mobile Radio with CEPT/GSM System Characteristics", Proc. of IEEE Globecom, pp. 26.3.1-26.3.5, 1988.
The CEPT/GSM system is quite different from the system proposed for use in the U.S. in that it employs a narrower time slot, partial response modulation Gaussian Minimum Shift Keying (GMSK), a wider bandwidth (200 KHz) and a higher data rate (270.8 Kbits/sec.). The narrower time slot typically permits the channel to be treated as being time invariant, the wider bandwidth implies a reduced fade depth and the higher data rate results in increased ISI. As a result, the receiver equalization requirements of the European and the proposed U.S. cellular systems are different.
An MLSE-VA equalizer for use with the proposed US digital cellular system is disclosed by the present inventors S. Chennakeshu, A. Narasimhan and J. B. Anderson in copending and commonly assigned U.S patent application Ser. No. 07/753,578, filed Sept. 3, 1991 entitled "Adaptive MSLE-VA Receiver for Digital Cellular Radio". This MLSE-VA technique is based on the approach described by G. Ungerboeck in "Adaptive Maximum Likelihood Receiver for Carrier Modulated Data Transmission Systems", IEEE Trans. Comm., Vol. COM-22, No. 5, pp. 624-636, May 1974, which is incorporated herein by reference. The novel modifications made to this receiver, to render it operational in the mobile channel, include: splitting the front-end matched filter into (a) a fixed transversal filter that is matched to the transmitted signal pulse shape and (b) into an adaptive transversal filter that uses a complex fast Kalman algorithm to obtain an initial estimate of the channel. The adaptive transversal filter employs a normalized least mean square (NLMS) algorithm for signal element updates and for relating an adaptation rate to the decision depth of the Viterbi algorithm. The complex Fast Kalman Algorithm described therein is an extension of the type taught by D. Falconer et al. in "Application of Fast Kalman Estimation to Adaptive Equalization", IEEE Trans. Comm. Vol. Com-26, No. 10, pp. 1439-1446, October 1978, which is incorporated herein by reference. The novel extensions made to Falconer's technique provide for use with complex input data and without requiring matrix inversions.
Another MLSE demodulation approach, incorporated herein by reference, is described by G. D. Forney in "Maximum Likelihood Sequence Estimation of Digital Sequences in the Presence of Intersymbol Interference", IEEE Trans. Info. Theory, Vol. IT-18, pp. 363-378, May 1972. Forney's approach uses the Viterbi algorithm with a squared metric that is based on an assumption that the additive noise in the receive signal, at the input to the maximum likelihood sequence estimator, is white and Gaussian. This is accomplished through use of a whitening filter at the input of the maximum likelihood sequence estimator.
Another equalization technique employs an equalizer based on an adaptive, fractionally spaced DFE and is disclosed by the present inventors S. Chennakeshu, A. Narasimhan and J. B. Anderson in copending and commonly assigned U.S. patent application Ser. No. 07/754,105, filed Sept. 3, 1981, entitled "Decision Feedback Equalization For Digital Cellular Radio".
Another equalization technique is based on an adaptive Lattice DFE (L-DFE). By example, in articles entitled "Adaptive Lattice Decision-Feedback Equalizers-Their Performance and Application to Time-Variant Multipath Channels", IEEE Trans. Comm. Vol. COM-33, No. 4, pp. 348-356, April 1985 and "A Generalized Multichannel Least Squares Lattice Algorithm Based on Sequential Processing Stages", IEEE Trans. Acoust., Speech, Signal Processing, Vol. ASSP-32, No. 2, pp. 381-389, April 1984, Fuyun Ling and John G. Proakis describe a Least Squares L-DFE and a gradient L-DFE. Advantages of L-DFEs are said to include numerical stability, computational efficiency, a flexibility in changing a length of the equalizer and an excellent capability for tracking rapidly time-variant channels. These two Ling and Proakis articles are incorporated herein by reference.
It is thus an object of the invention to provide an improved DFE receiver for a digital cellular radio system.
Another object of the invention to provide an Order Recursive L-DFE receiver for a digital cellular radio system that is suitable for use with the proposed U.S. cellular radio signal standard.
A further object of the invention to provide a receiver for a digital cellular radio system that operates in accordance with an improved Order Recursive L-DFE and that employs an adaptive technique for continuously determining an order of the L-DFE.